The present invention relates to a voice coil motor which is classified as a kind of a linear motor for conveying a head of an optical (or magneto-optical) disk drive or a magnetic disk drive, etc., and more particularly to a voice coil motor incorporating a shorted turn for causing the motor's control function to become uniform.
An optical head of an optical (or magneto-optical) disk drive or a magnetic head of magnetic disk drive is installed to move in the direction of the disk's diameter to seek information recorded in the disk. To make the head trace a desired track quickly and precisely, a motor having a quick response and constant control function is required.
FIGS.1, 2 and 3 illustrate simple and typical voice coil motors for use in conveying a head. The voice coil motor comprises a yoke member 10 forming magnetic paths, permanent magnets 20 and 20' mounted on the respective upper and lower portions 12 and 12' of the yoke member 10, maintaining air gaps G on both sides of central portion 11 of the yoke member, and a moving coil 30 wound slidably around the central portion 11 of yoke member 10. When current flows through moving coil 30, electromagnetic thrust works in a direction perpendicular to those of the current and the magnetic flux of permanent magnets 20 and 20'. Moving coil 30 moves along central portion 11 of yoke member 10 due to the thrust. The magnetic flux paths by permanent magnets 20 and 20' are shown in FIG.2, and the magnetic flux paths by moving coil 30 are shown in FIG.3. In other words, magnetic flux fm.sub.1 and fm.sub.2 generated from permanent magnet 20 or magnetic flux fm.sub.3 and fm.sub.4 generated from permanent magnet 20' are horizontally separated into left and right fields and flow in opposing directions within yoke member 10. Magnetic flux fc.sub.1 and fc.sub.2 generated by current flowing through moving coil 30 are vertically divided into upper and lower fields and each flux flows in a certain direction according to the direction of its current. Magnetic flux fm.sub.1 to fm.sub.4 of the permanent magnets constantly flow but magnetic flux fc.sub.1 and fc.sub.2 of moving coil 30 have varied strength and direction according to current. Thus, the density of the whole magnetic flux distributed within the yoke member varies by the movement of the moving coil. This brings out the variation of density of air gap's magnetic flux distributed in air gaps G between the central portion 11 of the yoke member 10 and each of the permanent magnets 20 and 20', in effect causing a problem by deteriorating the control function.
FIG.4 illustrates a voice coil motor having a conventional shorted turn to solve the above problem. Referring to FIG.4, the voice coil motor comprises shorted turn 40 made by coating a conductive plate having a certain thickness in central portion 11 of yoke member 10 in order to reduce the influence of the magnetic flux of the moving coil on the magnetic flux of the permanent magnet. That is, when the magnetic flux of moving coil 30 is varied, short circuit current is induced to the shorted turn and the magnetic flux of the moving coil is offset due to a secondary magnetic flux formed by the short circuit current. The technology of the shorted turn is described in detail in IEEE Transactions Mag., Vol. 25, No.4, 1989, pages 3073 to 3075.
Due to the operation of the shorted turn, such voice coil motor of the above-mentioned structure has less variation of the magnetic flux of the permanent magnet distributed in the yoke member and also constant density of air gap's magnetic flux distributed in the air gap between the permanent magnet and the central portion of the yoke member. However, since the shorted turn exists within the air gap, the air gap should be widened as much as the thickness of the shorted turn. This decreases the density of the air gap's magnetic flux. Accordingly, the voice coil motor having a shorted turn in the central portion of the yoke member has a weak thrust. To solve the problem, a permanent magnet of high efficiency must be used or the current of the moving coil must be increased, which, however, is undesirable because of the rise in the product's cost and its limit.